In known systems, for example closed circuit television systems, a camera unit such as a vidicon captures the image and then transmits it in analogue form to a station via coaxial cable Any processing that needs to be performed is usually done at the station. This is at a normally remote central location where with the provision of an analogue to digital converter (ADC) and computer any necessary processing functions are performed. The output from the camera may also be converted into digital form before being transmitted over the communication link. The bandwidth of the communication link places a further limitation and expense on this type of system.
It is also known to use motion detection software or hardware in security systems to indicate the presence of an intruder or of some disturbance in a given observed space. These know systems generally employ a remote camera which communicates via a communication link with a central observation and processing facility. The central processing facility performs the motion detection with a computer. The communication bandwidth required between the camera and the facility may need to be of the order of several MHz to provide analysis in real time as required for these types of situations. In addition available cameras only provide an analogue output requiring an analogue to digital converter to provide the digital format needed for processing by the computer.
A known imaging device which provides a digital output is a dynamic RAM having a transparent window as disclosed in U.S. Pat. No. 4,441,125 to Parkinson. This image sensor uses the light sensitive semi-conductor memory elements of the dynamic RAM to provide a black and white image or it can be employed to provide a variable grey scale output. This image sensor has been described for use in low cost applications such as robots and toys. It is much cheaper and of small size compared with other imaging devices such as vidicons and CCDs (charged coupled devices). The Parkinson specification describes various modes for operating the dynamic RAM as an image sensor. The sensitivity of the sensor can be controlled by varying the rate of scanning of the array or by changing the threshold value for the determination of the logic state of the cells of the RAM. By scanning the cells with a threshold value which follows a repeating sequence of voltage steps shades of grey can be determined for an image. This can also be achieved by scanning the cell arrays at various rates (periods).